Add ability to perform sign step with pre-generated key

[mjc4wilton]

I understand why this feature was originally removed, however it would help us significantly as we run our own private modpack and manually re-sign all addons with a key generated for each modpack version. Having the ability to either move away from dssign (ideal) or even just be able to build our own addons without having to redo them as well would be nice.

To prevent newer modders from using it incorrectly, I propose that it is something that can be done via a rhai script so I can hook into the release stages and sign the files there with a necessary key. Additionally, having a hemtt util to just re-sign generic PBOs would be nice for when we resign other mods to minimize the number of tools we are working with, however I can understand if this is cut.

[BrettMayson]

https://github.com/SynixeContractors/resign

I'd rather polish that project and have people use it for this purpose, as that's what I use it for.

It resigns every mod with its own new key anytime they change, so it's Swifty friendly

[BrettMayson]

That repo was released as https://github.com/BrettMayson/Pallas